Plan the composition

You know the problem and have the material. Now decide how best to arrange it. This is a plan, not a build, keep it short and confirm it before creating anything.

First, choose the primitive

Before deciding single-skill-vs-plugin, decide what kind of context each part is, because it is easy to default everything to skills. See references/choosing-the-shape.md.

• If the agent should follow a workflow when a relevant task comes up, use a skill.
• If the agent should always obey a convention, unprompted, use a rule.
• If behavior must run deterministically at an agent lifecycle event, use a hook. Prefer cross-agent hooks; use nativeHooks only when the event or data is agent-specific.
• If the agent needs live tools or data, use an MCP server.

Watch for always-on-convention language: "the agent keeps getting X wrong", "every time", "it never follows our...". That is usually a rule (or a rule plus a skill), not a skill alone. Do not push a conventions request toward skills by default.

Decide the shape

Default to the smallest thing that solves the problem.

• A single skill — for a simple, self-contained workflow. In Tessl a skill is already a minimal plugin (it gets a plugin.json), so this is not a lesser option; it is the right one when the problem is simple.
• A plugin with several primitives — when the problem has distinct parts (decompose), related skills belong together, or it mixes model guidance with deterministic behavior or live capabilities.
• A plugin with a hook — when a command must run automatically at a lifecycle event. Use the generic tier for agent-independent behavior and the native tier only as an agent-specific escape hatch.
• A plugin with an MCP server — when the agent needs a tool or live data, not just instructions. Add one only when it clearly earns its place.

Write the plan

In a few lines, state: the problem, the artifacts, and the proposed shape, which primitive each part becomes (skill, rule, hook, MCP server, or command), and whether anything needs decomposing. For a hook, state the lifecycle event and whether the generic or native tier is justified. Explain the why, especially if you are proposing to split a big skill, encode a convention as a rule, or use an agent-specific hook. Confirm with the user, or make the call and tell them your reasoning.

Hand off

Pass the confirmed plan to build-composition.

When to stop

Stop when there is a confirmed composition plan.